Many applications incorporate a tunable integrated circuit capacitor. Micromachined electro-mechanical devices (MEMs) provide a way to construct a tunable capacitor within an integrated circuit. An important parameter for a capacitor is the quality factor (Q). To achieve a high Q, a capacitor should have a low parasitic series resistance, as given by the equation Q=1/(2πfRC), where R is the parasitic series resistance. A high-Q capacitor is desirable in many applications. For example, low-noise radio frequency (RF) voltage controlled oscillators (VCOs) need a resonant device with a high Q because the phase noise of an oscillator is proportional to 1/Q2, where Q is the overall Q of the resonator (W. P. Robins, “Phase Noise in Signal Sources: Theory and Applications.” Stevenage, U.K: Peregrinus, 1982, pp. 49-53). Also, high dynamic range filters need a high-Q resonator because the dynamic range of the filter is proportional to Q2 (S. Pipilos et al., “A Si 1.8 GHz RLC Filter with Tunable Center Frequency and Quality Factor,” IEEE J. Solid-State Circuits, vol. 31, pp. 1517-1525, October 1996).
FIG. 1 illustrates a conventional MEM tunable interdigitated capacitor 100. The fixed fingers 110 attached to the fixed block 115 serve as a first electrode of the tunable interdigitated capacitor 100. The movable fingers 120 attached to the movable block 125 serve as a second electrode of the tunable interdigitated capacitor 100. The movable fingers 120 do not move relative to the movable block 125, but rather move with it. The movable block 125 is attached to a system of springs 130, which are typically hundreds of micrometers (μm) long but only about a few μm wide.
A control voltage is used to increase the overlap between the movable fingers 120 and the fixed fingers 110. One terminal of the control voltage is electrically coupled to the fixed block 115. The other terminal of the control voltage is fed into the movable block 125 via one or more of the springs 130 attached to the movable block 125. Increasing the magnitude of the control voltage causes the movable fingers 120 to move between the fixed fingers 110, as shown by the arrows 135 in FIG. 1. The electrostatic force resulting from the control voltage works against the force from the system of springs 130. Varying the magnitude of the control voltage alters the overlap between the fixed fingers 110 and movable fingers 120, and hence varies the capacitance. In this fashion, a tunable interdigitated capacitor is achieved using MEM technology. The tunable interdigitated capacitor 100 has a substantial range in capacitance because the area of overlap between the fixed fingers 110 and movable fingers 120 has a large range.
The capacitance is a function of the overlap between the fixed fingers 110 and the movable fingers 120. The fixed fingers 110 are coupled to one terminal of the capacitance voltage through the fixed block 115. The movable electrodes 120 are coupled to the other terminal of the capacitance voltage thorough the movable block 125 via one or more of the springs 130. Significantly, the springs 130 must be narrow to provide a suitable spring coefficient. A typical width is a three μm. Moreover, the springs 130 must be relatively long. A typical length is hundreds of μm. Therefore, the springs 130 through which current flows to the capacitor have a relatively high resistance. Typically, the resistance is on the order of one ohm, which is relatively high for this type of circuit and considerably degrades the Q.
Thus, one problem with conventional MEM tunable interdigitated capacitors is that such capacitors have a relatively high parasitic resistance that lowers the Q.